1. Field of the Invention
The present invention relates to a method for catalytically purifying an exhaust gas, and more particularly, it relates to a method for catalytically purifying an exhaust gas which efficiently purifies NO.sub.x (nitrogen oxides) in an oxygen excess exhaust gas, i.e., an exhaust gas containing oxygen in an amount excessive to the oxygen amount necessary for completely oxidizing reductive substances such as carbon monooxide, hydrogen and hydrocarbons, etc., contained in the exhaust gas.
2. Description of the Related Art
Known in the art are a large number of catalysts for purifying exhaust gases, which catalysts purify an exhaust gas by effecting, at the same time, an oxidation of carbon monooxide (CO) and hydrocarbons (HC) and a reduction of nitrogen oxides (NO.sub.x) in the exhaust gas, for purifying the exhaust gases of automobiles. Typical examples of such a catalyst include those prepared by coating a .gamma.-alumina slurry on a refractory support, such as cordierite, and calcining the coating and supporting thereon a noble metal, such as Pd, Pt and Rh.
Nevertheless, the performances of the above-mentioned catalysts for purifying exhaust gas are greatly influenced by the air-fuel ratio of the engine, and when a large amount of oxygen is present on the lean side, i.e., a lean mixture after combustion, although the oxidation actively proceeds little reduction of NO.sub.x occurs. Conversely, on the rich side with a smaller air-fuel ratio, the amount of oxygen in the exhaust gas becomes smaller, and thus little oxidation occurs but a large reduction of NO.sub.x occurs. Recently, in response to a demand for lower fuel costs, a lean burn running has been practiced which effects a combustion with an oxygen excess mixed gas during normal running, and there is a need for a catalyst which can sufficiently purify NO.sub.x even during a lean burn running.
Under the above circumstances, there has been proposed a NO.sub.x purifying catalyst having Cu, Co or Pt supported on a zeolite carrier. Among these catalysts, for example, the Pt/zeolite catalyst at a catalyst inlet gas temperature of about 200.degree. C., the Cu/zeolite catalyst at about 430.degree. C., and the Co/zeolite catalyst at about 490.degree. C., respectively, purify NO.sub.x at a high efficiency, as shown in FIG. 1.
As described above, with the catalyst of the prior art having Cu, Co or Pt supported on a zeolite carrier, NO.sub.x cannot be purified at high efficiency at around 300.degree. C., which is the exhaust temperature during a steady lean burn running of the engine.